PROJECT SUMMARY/ABSTRACT Loss of function mutations in the genes PINK1 and PRKN (Parkin) are the most common causes of early-onset Parkinson?s disease (EOPD). Under conditions of stress, the encoded proteins, a kinase and a ubiquitin ligase, jointly identify and tag damaged mitochondria with phosphorylated ubiquitin (pS65-Ub) for selective degradation via autophagy and lysosomes (mitophagy). This mitochondrial quality control (mitoQC) pathway is thought to prevent the accumulation of damaged mitochondria and the consequent cell death. There has been debate over the years whether heterozygous mutations in these recessive genes influence susceptibility to late-onset PD, but the question remains. Given that certain heterozygous mutations have been associated with seudo-dominant inheritance and subclinical phenotypes, it may well be that assessing the effect of specific variants rather than on the gene level will provide more insight. We hypothesize that reduced functions of the pathway may at least modify risk and contribute to heterogeneity in onset, progression and clinical phenotypes of late-onset sporadic PD. However, critical threshold levels and pathomechanisms are unclear. We propose to study heterozygous PINK1 variants, determine their enzymatic activity by structure and function in isogenic cells and thereby define their potential liabilities or protection. This will help reassess clinical-genetic correlations and will inform the development of more accurate animal models and targeted drugs. Further, based on the sequence of events, we suggest that mitoQC can be impaired at multiple levels of the pathway (mitochondria ? autophagy ? lysosomes) and by distinct mechanisms. Alpha-synuclein, the major constituent of Lewy bodies found in PD, is known to affect all of these organellar systems, but the underlying mechanisms remain elusive. We propose a bi-directional cross-talk, where alpha-synuclein induces dysfunctions in mitoQC, but these can further contribute to processing, aggregation, and toxicity of alpha-synuclein, and we will test this in neurons in culture in human post-mortem brains of pathological proven Lewy body disease (LBD). Moreover, we will quantify pS65-Ub levels in large cohorts of LBD and exploit levels of this mitophagy tag as a quantitative endophenotypic trait. This will allow us to study genetic associations in order to identify novel genetic modifiers and to generate a polygenic risk scores for genes involved in mitochondria or autophagy/lysosomal functions. In the future this may help to predict risk and progression in clinical series.